Page 89 - An Introduction to Microelectromechanical Systems Engineering

68 Processes for Micromachining

Ultrasonic Machining

In ultrasonic machining, also known as ultrasonic impact grinding, a transducer
vibrates a tool at high frequency (20–100 kHz). The tip of the tool is pushed against
the workpiece as a slurry of water or oil and abrasive particles, such as boron car-
bide, aluminum oxide, or silicon carbide, is flushed across the surface. There are
several mechanisms for removal of material: The tool vibration directly hammers
particles into the surface, as well as imparting a high velocity to other particles,
both of which chip away at the workpiece. Cavitation erosion and chemical action
can also contribute. The microscopic chips are carried away by the slurry. As the
tool moves slowly into the workpiece, a hole with vertical sidewalls is created. An
array of tips can drill many holes at the same time; Figure 3.28 shows examples in
several materials. The hole shape matches that of the tool and can be round, square,
or other.
Ultrasonic machining can be performed on hard, brittle materials (with a Knoop
hardness above about 400) such as glasses, ceramics, diamond, and silicon. The
minimum hole diameter is about 150 µm. At the other extreme, holes over 100 mm
have been machined. For small holes, the maximum aspect ratio is about five,
increasing to over 15 for holes several millimeters in diameter. With tolerancing, the
size accuracy of 1-mm holes is typically ±50 µm, improving to ±25 µm for larger
holes. Hole depth can be over 10 mm.

Combining the Tools—Examples of Commercial Processes

The sequence in which various processes from the toolbox are combined determines
a unique microfabrication process. The process may be specific to a particular
design or may be sufficiently general that it can be used to fabricate a range of
designs. This section describes four example fabrication processes that are generic in
their nature and used today in manufacturing at a number of companies and
commercial foundries.

Figure 3.28 Photograph of ultrasonically drilled holes and cavities in glass (clear), alumina
ceramic (white), and silicon (shiny). All of the holes in a single substrate are drilled simultaneously.
(Courtesy of: Bullen Ultrasonics, Inc., of Eaton, Ohio.)